The present invention relates to laminar heating devices, and more particularly, to heating devices with temperature self-regulating properties. Laminar heating devices having resistive elements containing conductive carbon or metal powders dispersed in a polymer matrix are well known in the art. For example, U.S. Pat. No. 2,715,668 discloses an electrically conductive film panel heater having a resistive element fabricated from silver flake, antimony oxide, and graphite dispersed in an alkyd resin and xylene. U.S. Pat. No. 3,287,684 discloses an electrical heating device made from a conducting ink of a mixture of polyester resin, graphite and a solvent. U.S. Pat. No. 3,457,537 discloses a flexible resistance element film of a tetrafluoroethylene film coated with a formulation of a phenolic resin, conductive carbon black and acetone. U.S. Pat. No. 3,878,362 discloses an electrical heater having a laminated structure. The resistive element has an insulating polymeric film coated with an electrically conductive elastomeric material containing conductive carbon black dispersed in a heat-resistant elastomer. U.S. Pat. No. 4,071,736 discloses a defrosting mirror made by painting a resistive coating with graphite particles dispersed in a solvated fluoroelastomer polymer onto a primed glass surface.
The polymer component of the foregoing electrically conductive coatings are all non-crystalline or very low in crystallinity. As a consequence, the resulting heaters are constant wattage devices requiring the added expense of a thermostat in order to attain temperature self-regulation and overheat protection.
Non-laminar heating devices which have temperature control, so-called temperature self-regulating heating devices, are well known in the art. Such heating devices include, for example, two discrete, spaced-apart elongated conductor wires joined in physical and electrical contact by an extruded, cross-linked, solid polymer of carbon black particles dispersed in a crystalline polymer matrix. These heating devices are essentially heating cables, and are neither low-cost nor laminar heating devices.
Laminar temperature self-regulating heating devices are disclosed in U.S. Pat. Nos. 4,761,541 and 4,777,351. The resistive element described in these patents is an extruded and irradiation cross-linked thin sheet consisting of carbon black dispersed in a polyvinylidene fluoride polymer. Electrodes are screen printed on one surface of the sheet using commercial conductive ink of silver particles dispersed in a solvated elastomer. This method requires melt-extrusion and irradiation cross-linking, both of which are relatively expensive procedures. U.S. Pat. Nos. 4,761,541 and 4,777,351 disclose the use of crystalline polymers, ethylene/ethyl acrylate copolymer and polyvinylidene fluoride, as components of compositions which exhibit temperature self-regulating properties. Other polymers disclosed are thermoplastic crystalline polymers such as olefin polymers, including homopolymers, particularly polyethylene and polyalkenamers obtained by polymerizing cycloolefins; copolymers of two or more olefins, and copolymers of one or more olefins, e.g., ethylene or propylene, with one or more olefinically unsaturated comonomers, preferable polar comonomers, e.g., vinyl acetate, acrylic acid, methyl acrylate and ethyl acrylate. Also disclosed are fluoropolymers, in particular, polyvinylidene and copolymers of ethylene with tetrafluroethylene and/or a perfluoro-alkoxy comonomer. While these polymers have properties which enable compositions containing them to be extruded, calendered and molded, they have poor solubility in conventional solvents at ambient temperatures so that it is difficult to formulate solution coatings or inks using them.
As disclosed in U.S. Pat. No. 3,793,716, crystalline polymers can be dissolved in some high boiling solvents at temperatures above the crystalline melt temperature of the polymers. However, such solutions are hazardous and therefore extremely difficult to handle in coating operations.
It is an object of the present invention to provide a low-cost temperature self-regulating composition and a heating element containing the composition using resistive inks printed onto an insulating polymer film, the film being affixable to a temperature-sensitive device for control of the temperature of that device.